This invention relates generally to electric motors and, more particularly, to stator yokes for electric motors.
An electric motor includes a stator that rotatably supports a rotor. The stator may include a plurality of permanent magnets connected to a stator yoke. Typically, the stator yoke is a tubular member located between the magnets and a stator shell. The stator yoke and the magnets are secured to the stator shell with adhesive, clips, or other attachment devices. The rotor is mounted within a cavity formed by the magnets and includes a plurality of windings.
An air gap extends between the rotor and the magnets. It is desirable for the air gap to be substantially uniform since favorable operating characteristics are then achieved. However, a uniform air gap is not easily provided since variations in the stator yoke can lead to variations in the air gap. Typically, to reduce variations in the stator yoke, a final machining operation is utilized. For example, it is desirable for the stator yoke to have an outer diameter with a final tolerance of about 0.002 inches and an inner diameter with a tolerance of about 0.010 inches. The final machining operation adds additional expense to the cost of fabricating the electric motor and increases the time for fabrication.
Accordingly, it would be desirable to provide a stator yoke that was easy to fabricate and that provided a substantially uniform air gap between the magnets and the rotor. Additionally, it would be desirable if the stator yoke was relatively inexpensive to fabricate and did not require a final machining operation.
These and other objects may be attained by a stator yoke assembly that includes a plurality of layers of similarly sized sheets of metal. The layers are formed into a substantially cylindrical shape with a gap extending between ends of the sheets. One, two, or more sheets can form a single layer.
In one embodiment, the stator yoke is positioned within a shell of an electric motor. A plurality of permanent magnets are attached to the stator yoke and the shell. The layers of the stator yoke are concentric and the gaps in each layer are aligned with gaps in adjacent layers to form an opening that extends radially through the stator yoke. The presence of the gap between ends of the metal sheets eliminates the requirement that the metal sheets have limited tolerances since the stator yoke layers can be formed to have limited tolerances by adjusting the gap in each layer. The stator yoke gaps are beneficial to the demagnetization performance of the motor. Since the layers are concentric and formed from similarly sized sheets of metal, the openings in the stator yoke decrease in size from an outer stator yoke layer to an inner stator yoke layer.
The gaps in the layers of the stator yoke allow the layers to be formed with limited tolerances without requiring a final machining step to provide a limited tolerance to the metal sheets. The fabrication expense of the metal sheets is thus reduced without reducing the performance of the electric motor. In addition, the stator yoke is easy to assemble.